This invention relates to power control circuitry and, more particularly, to circuitry functioning as a time delay/holding relay for controlling input power to a device of the type requiring a warm-up period prior to operation.
Many electrically powered devices require a warm-up period after being initially turned on in order to become operative. In particular, this applies to electric devices utlizing vacuum tube elements having heater filaments. For example, microwave ovens utilize magnetrons for generating power at microwave frequencies. These magnetrons have heater filaments and require an initial warm-up period. Many times during the use of such a device, it is turned off momentarily only to be turned on again immediately thereafter. For example, when cooking using a microwave oven, a first dish may be cooked, the oven turned off for removal thereof and a second dish may be placed therein, the oven then being turned on again, with the period during which the oven is turned off being only a matter of seconds. It would be desirable to incorporate in the oven controls a timing device whereby when the oven is turned off, the filaments of the magnetron remain on for a predetermined period, say 60 seconds, so that the oven may be turned on again and made operative without an additional warm-up period. In the past, electromechanical time delay/holding relays have been utilized to perform this function. The use of such relays has not been entirely satisfactory however, electromechanical devices tending to have inherent reliability problems. It is therefore desirable to replace the electromechanical relay with solid state circuitry which is more reliable and trouble free.